Let's talk about blood
pressure homeostasis. And what homeostasis
means is balance. So how is it that our body
is able to create balance for our blood pressure? So this is the
heart, and we've got branches of the aorta
coming off of it. I haven't been drawing these
branches every single time, but this time I think
it's quite helpful to see. We've got here the
left brachial artery, going out to the
left arm, and we've got the left
carotid artery here. And again, I'm
writing left and right from the perspective of the
person whose heart this is. And you've got here the
right carotid artery and the right brachial artery. So this is blood going
to the right arm. And we've got blood
going to the right neck. One interesting thing, if you
look at the right carotid, is that it bulges right
here- in fact, both sides do. And they bulge right
before they split. And so that bulge is actually
called the carotid sinus, right here. And we call it that because
a sinus is any cavity. And so this is the
right carotid sinus, and this is the
left carotid sinus. Another spot I'm going to
talk about in this video is the aortic arch,
which is right there. So these three spots--
the two carotid sinuses and the aortic arch-- are
really, really, interesting, and actually they're very
important for learning about how it is that our body
is able to create balance in our blood pressure. So at the top I drew
kind of a blow-up version of the carotid sinus, and at
the bottom is the aortic arch. And if you look closely
under a microscope, you'd see nerve endings on
the outer layer of the vessel. And so these nerve
endings basically join up and form a nerve, and
these on the carotid sinus do the same thing. And they are basically
going to form two large nerves that go off. And they send
information about what's happening in the blood vessel,
specifically about stretch. So as blood is pulsing
through this vessel right here, this carotid sinus, or as
it's pulsing through the aorta, even, that wall is
being stretched out. And as it gets stretched
out, these nerves-- they're very special nerves, they're
called baroreceptors. Baro, meaning pressure, and
they're receptors for pressure, so they're baroreceptors. These baroreceptors are
feeling the effects of stretch. And what they do is,
they send a signal down the nerve that
tells the brain how much stretch is happening. And so if this is the brain,
let's say, we have here your midbrain,
these nerve endings are going to actually
go here, and tell the brain--
communicate information about how much stretch is
happening in those vessels. Now we know that the more
pressure is in the vessel, the more it's going to stretch. So follow with me
in a little example. So let's say we have
blood pressure here, and let's say I have my blood
pressure of 115 over 75. And in green, I'll write
action potentials per minute. So what happens is that as my
blood pressure is 115 over 75, those nerves are feeling a
certain amount of stretch, whatever that amount is, and
they're going to send a signal. Not just one, but they're
going to send a handful. So let's say they
send 10 signals. 10 signals. I'm going to draw them out here. 5, 6, let's say 7, 8, 9, and 10. 10 in one minute. And actually, let's just
imagine that both nerves are doing this. So they're doing 10 per minute. Well, that's a pretty
normal number, let's say. And this, over
time, becomes what the brain regards as
my normal set point. The brain starts to assume that
if 10 action potentials are fired per minute, then
that's pretty normal for me. So it regards this as
my normal set point. Now if my pressure
goes up-- let's say that I'm running
late to an exam, or something happens
that really worries me, and my pressure
goes to 140 over 90. Now I have hypertension. And this is my new pressure. This would be much
higher than normal. So my body would register
this and my nerves would start firing, let's
say at 30 times per minute. So if they're firing
at 30 times per minute, then my body is
thinking, or my brain is thinking, well, that's
higher than normal. So this must be high. It regards this as high. And on the flip side,
let's say that, you know, I have-- let's say I cut my
arm and I lose a lot of blood, and my blood pressure
starts to fall. My stretching is going to
happen less than before. So it's going to send less
action potentials per minute, maybe only seven per minute. And again, my midbrain
is going to get seven little green arrows
per minute, seven action potentials per minute. And it's going to think,
well, that's very odd. Before, it was 10 per minute. So this represents a
fall in blood pressure. So now you have high
blood pressure in pink, and a fall in blood
pressure in blue. So what exactly
can the brain do? What can the brain do to help
normalize or create balance? So let me write that
in red over here. Let's write response,
question mark. So the body has a
couple of strategies, and they're basically
summed up in the autonomic nervous system. And there's two major branches
of your autonomic nervous system, or two parts
to it, let's say parts. One is called the sympathetics,
almost like sympathy. And the other is called
parasympathetics. They're very similar
words, except the word para is in front of this one. And I want you to remember
now that there's a formula. And I'm going to write
that formula down here, just to remind us that pressure
equals flow times resistance. And additionally, I
want you to remember that flow-- this
one right here-- is going to be related to
stroke volume times heart rate. So if I can do anything--
if my body can do anything to raise a stroke volume or the
heart rate, or the resistance-- then my pressure will go up. And vice versa, if I can drop
the stroke volume, or heart rate, or resistance, then
my pressure will go down. So what the
sympathetics do is they have an effect on the
heart and the vessel. And these blood vessels
are all over the body, not just the carotid
sinus or the aortic arch, I'm talking about
all blood vessels. And so the sympathetics are
going to, for the heart, they're going to
increase the heart rate. And they're going to
increase the stroke volume. And the parasympathetics
do the opposite. They actually drop
the heart rate and drop the stroke volume. And the way that they do that--
the heart rate is controlled by how many beats
you get per minute. Obviously, that's
the heart rate. And the sympathetics are going
to cause the heart cells that control that to work faster,
and the parasympathetics will slow them down. And for the stroke
volume, the sympathetics force the heart to
contract harder. And then you have more
volume of blood gushing out with every beat. And the parasympathetics make
the heart work less forcefully, so you have less blood
gushing out with every beat. And the sympathetics--
finally, they actually cause vasoconstriction. And, you guessed it,
the parasympathetics do the opposite. So they cause vasodilation. And vasoconstriction and
vasodilation basically mean whether the artery
stays open or closes down. So for the sympathetics,
the arteries and arterioles, primarily, mostly
it's the arterioles, they start to get smaller. And as they get smaller,
that increases resistance. And for the
parasympathetics, they will cause the arterioles
to get bigger, to dilate. And that will cause
the resistance to fall. So taking a quick
peek at our equation that I wrote out for
you on the right, you can see that the
sympathetics basically do everything that will help
to increase the pressure. So if you have a pressure,
again, of 140 over 90, then what will
happen is your body will see that as a
high pressure and will try to get the
parasympathetics to be active-- will activate all the
parasympathetic nerves. And if your pressure is
low, if it's 90 over 60, then the body is
going to respond by getting all the
sympathetics to react. You see how that works? And of course, if your pressure,
let's say, is 115 over 75, and the baroreceptors
are firing, you know, the usual
10 times per minute, then there should be
really overall no response. So here you would
really get no response, because the body is
thinking, well everything is already balanced, there's
nothing for it to do. So this is how the
body is able to control blood pressure in a rapid way. So that's the final
point I want to make. That the input here,
the baroreceptors, these are nerves. And the autonomic nervous
system, obviously these are nerves. So the information going
in, is the baroreceptors. The information going out is
the autonomic nervous system, and all of this is
happening rapidly. This is all very rapid. And when I say rapid, I mean
on the order of kind of seconds to minutes. So within seconds to minutes,
this response can happen. So this is a fantastic
example of how your body can take in
information really quickly, and really respond quickly to
help keep your blood pressure balanced.